T cells are central in the development of adaptive immunity. T cell development and activation are stringently regulated and perturbation of them will cause disregulated immune responses leading to diseases like leukemia, lymphoma, AIDS, and autoimmune diseases. Naive T-cell activation requires at least two signals that are provided by co-stimulation of T cell receptor (TCR) and co-stimulatory receptors like CD28. Precisely how these two signals are transduced and integrated to cause T cell proliferation, cytokine production, and effector cell differentiation remains unclear. The long-term goal is to determine the molecular mechanisms of signal transduction and integration in T cells. To achieve this goal, the investigator will study the c-Jun N-terminal kinase (JNK) cascade which they showed to require co-stimulation of TCR and CD28 for its activation . The JNK cascade regulates the activities of many transcription factors such as AP-1, a key factor required for T cell function. This cascade is the only known kinase cascade studied so far to require the TCR- and CD28-mediated signals for activation but the mechanism for its activation is unknown. Recently the investigator cloned human MEKK2 gene that encodes a protein kinase that acts between the receptor signals and JNK in T cells. In this project he will determine the mechanisms of the JNK cascade activation in T cells by focusing on MEKK2. In Specific Aim 1, he will determine how MEKK2 specifically activates the JNK cascade in T cells and how it activates T cell gene expression. In Specific Aim 2, he will investigate the molecular mechanisms of MEKK2 activation by T-cell co-stimulatory signals and determine whether TCR and CD28 signals integrate to MEKK2. In Specific Aim 3, will study the functions of MEKK2 and the JNK cascade in T cell development and activation by using transgenic mouse models. In addition to gaining insight into T cell signal transduction, the outcome from this study may have broader implications. First, MEKK2 is a member of the growing MAPK kinase kinase (MAP3K) supergene family, which is highly conserved from yeast to mammals. Studies on MEKK2 will provide conceptual and material resources for studying other MAP3Ks. Second, this study is clinically relevant, as abnormalities of the JNK cascade have been observed in T cell leukemia. The proposed studies will provide clues about the causes and potential cures for T cell leukemia.